Mineral oil lubricants including turbine oils are derived from various crude oil stocks by a variety of refining processes. Generally, these refining processes are directed towards obtaining a lubricant base stock of suitable boiling point, viscosity, viscosity index (VI) and other characteristics. Generally, the base stock will be produced from the crude oil by distillation of the crude in atmospheric and vacuum distillation towers, followed by the separation of undesirable aromatic components and finally, by dewaxing and various finishing steps. Because aromatic components lead to high viscosity and extremely poor viscosity indices, as well as poor oxidation stability in the finished product, the use of asphaltic type crudes is not preferred as the yield of acceptable lube stocks will be extremely low after the large quantities of aromatic components contained in such crudes have been separated out; paraffinic and naphthenic crude stocks will therefore be preferred but aromatic separation procedures will still be necessary in order to remove undesirable aromatic components. In the case of the lubricant distillate fractions, generally referred to as the neutrals, e.g., heavy neutral, light neutral, etc., the aromatics will usually be extracted by solvent extraction using a solvent such as furfural, N-methyl-2-pyrrolidone, phenol or another material which is selective for the extraction of the aromatic components. If the lube stock is a residual lube stock, the asphaltenes will first be removed in a propane deasphalting step followed by solvent extraction of residual aromatics to produce a lube generally referred to as bright stock. In either case, however, a dewaxing step is normally necessary in order for the lubricant to have a satisfactorily low pour point and cloud point, so that it will not solidify or precipitate the less soluble paraffinic components under the influence of low temperatures.
A number of dewaxing processes are known in the petroleum refining industry and of these, solvent dewaxing with solvents such as methylethylketone (MEK), a mixture of MEK and toluene or liquid propane, has been the one which has achieved the widest use in the industry.
The catalytic dewaxing process operates by selectively cracking the normal and slightly branched paraffins to produce lower molecular weight products which may then be removed by distillation from the higher boiling lube stock. The catalysts have usually been zeolites which have a pore size which admits the straight chain, waxy n-paraffins either alone or with only slightly branched chain paraffins but which exclude more highly branched materials and cycloaliphatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38 and the synthetic ferrierites have been proposed for this purpose in dewaxing processes, as described in U.S. Pat. Nos. 3,700,585 (Re 28398); 3,984,938; 3,933,974; 4,176,050; 4,181,598; 4,222,855; 4,259,170; 4,229,282; 4,251,499; 4,343,692, and 4,247,388. A dewaxing process employing synthetic offretite is described in U.S. Pat. No. 4,259,174. Processes of this type have become commercially available as shown by the 1986 Refining Process Handbook, Hydrocarbon Processing, September 1986, which refers to the availability of the Mobil Lube Dewaxing Process (MLDW). Reference is made to these disclosures for a description of various catalytic dewaxing processes.
Although these catalytic dewaxing processes are invariably carried out in the presence of hydrogen, it is not necessary for the stoichiometry of the dewaxing process which, as noted above, proceeds by a shape-selective cracking mechanism. For this reason it is not necessary for the catalyst to include a hydrogenation component although one may be included in order to improve catalyst reactivation. The hydrogen serves to extend catalyst life during each dewaxing cycle. The effluent from the dewaxing reactor includes olefins which have been produced by the cracking reactions and in order to stabilize the product, a hydrotreating step is carried out after the dewaxing to saturate lube boiling range olefins and, depending upon the hydrotreating conditions, to saturate aromatics remaining in the product stream as well as to remove heteroatom impurities, principally sulfur and nitrogen and various color bodies. A process for hydrotreating a catalytically dewaxed lube product is described in U.S. Pat. No. 4,181,598.
Turbine oils are a special class of lubricants which require exceptional oxidation stability over extended periods of time.
The exceptionally stringent product specifications associated with turbine oils are necessary because of the severe conditions associated with their use. Turbine oils are expected to last the life of the turbine. This involves years of continuous operation at moderately elevated temperature, and in the presence of air, water and metals. The conditions are not at all like those in an automobile. Good survey articles on the special problems of turbine oils are presented in:
1. Control of Turbine Oil Degradation During Use, M. J. Den Herder and P. C. Vienna, Lubrications Engineering, 37 (2), February 1981, and
2. Evaluation and Performance of Turbine Oils, G. H. von Fuchs et al, Industrial and Engineering Chemistry, Vol. 13, No. 15, both of which are incorporated by reference.
An additional indication of the severe uses to which turbine oils are put may be taken from the following standardized test methods used to define good turbine oil properties.